The Collaborative International Dictionary of English v.0.48:

Manganous \Man"ga*nous\, a. (Chem.)
   Of, pertaining to, designating, those compounds of manganese
   in which the element has a lower valence as contrasted with
   manganic compounds; as, manganous oxide.
   [1913 Webster]

   Manganous acid, a hypothetical compound analogous to
      sulphurous acid, and forming the so-called manganites.
      [1913 Webster]